most LEDs hooked up to AC current don't have filter capacitors to smooth out the polar flip flop, due to the fact that most people don't even notice the change. AC runs at 60Hz, which can be seen by any motion while looking at the light. instead of being "full on", the LED is turning off, on, off, on 60 times per second, just at the edge of our perception. this works while eating ice and using an electric toothbrush, because they both send vibrations to our eyes, allowing us to see the breaks between the AC flops. an easier way to do this is to turn your head rapidly (or spin in a chair) and you can perceive the effect. also, in said example, you could just pick the clock up (leaving it plugged in, of course,) and move it about rapidly (within the length of the cord, obviously).